The pulp industry especially in Scandinavia uses both softwood and hardwood as a raw material. The most important tree varieties used in pulping are pine, spruce and birch. In the pulping process wood is chipped into small particles which are then cooked in the presence of alkaline liquor, such as sodium sulfide and sodium hydroxide, at a temperature of about 150° C. to 170° C. During cooking the inherent adhesive of wood, lignin, dissolves in the aqueous solution and the fiber material is released and recovered as a raw material for paper.
When softwood is cooked, lignin and other extractives in the wood are released relatively easily. When hardwood is cooked, the extractives are not released as easily as from softwood because of the different chemical structure of softwood and hardwood and, thus, part of the extractives remains in the fibers. The extractives remaining in the fibers cause problems in the production of paper. The extractives cause problems especially when birch is used in the pulping.
During cooking the sodium salts of carboxylic acids, fatty acids and rosin acids wash out the extractives from cellulose. Softwood contains both fatty acids and rosin acids and the extractives are removed easily during cooking. Hardwood like birch, on the other hand, contains only fatty acids and no rosin acids at all. The fatty acids in birch are not able to wash out the extractives from birch during the pulp production. Fatty acids are mainly in an esterified form in the wood, whereas rosin acids exist as free acids. Fatty acid esters are degraded during the cooking process. Free fatty acids form sodium salts which are water soluble and act like detergents. However, in the case of birch the extractives are not washed out in the same way.
The problem with birch cooking can be avoided by using crude tall oil obtained from softwood cooking. The fatty acids and rosin acids of the crude tall oil improve the removal of the extractives from birch since they act as surfactants.
Textbooks in the technical field of the present invention teach that it is usual to add crude tall oil or black liquor soap to birch cooking in order to decrease the amount of extractives in the pulp. This is mentioned e.g. in the textbook of the Finnish Paper Engineers' Association called: Puumassan valmistus II osa 2, page 1341 (Production of pulp II, part 2). The addition of crude tall oil or black liquor soap both improves the separation of soap and the quality of pulp in the cooking of birch.
The prior art methods, however, are not sufficiently effective in removing the extractives and thus some of the extractives remain still in the pulp after cooking. Some of the agents used in the prior art are also very expensive and the use thereof raises cost. Some agents, on the other hand, are difficult to dispose of after use which make them less practical in use. Thus, there is a need for a more efficient way of removing extractives from hardwood which is technically feasible and also cost-effective in use.
It has now been found that certain wood cooking aids provide an effective way to remove extractives during cooking. Effective removal of the extractives ascertains pure pulp which does not cause problems in paper production.